1. Field of the Invention
The present invention relates to a range finding device for detecting a distance of a subject by an optical triangulation method, and more particularly to an improved range finding device free from displacement of a lens optical axis as caused by environmental conditions.
2. Description Related to the Prior Art
Recent photographic cameras and video cameras are equipped with an autofocus system for automatically focussing a taking lens. This autofocus system is constructed of a distance measuring device for measuring a distance of a subject and a lens setting mechanism for setting a taking lens to a position suitable for the subject distance. As a distance measuring device, a range finding device is widely used which measures a distance of a subject by an optical triangulation method. Such a range finding device includes an active type device and a passive type device. The active type device has a light projector for projecting light toward a subject and a light receiver for detecting light reflected from the subject, the light projector and light receiver being disposed spaced apart by a predetermined length of a base line. The passive type device has two light receivers spaced apart by a predetermined base length.
FIG. 7 shows an autofocus system having a passive type range finding device. Two lenses 3a and 3b having the same specifics are mounted with their optical axes being disposed in parallel and with the distance between two optical axes being set to a lens base length B. Mounted on the focussing plane of the lenses 3a and 3b are line image sensors 5a and 5b each having a plurality of pixels disposed in the direction along the base line. Representing the focal length of each lens by F, a distance of a subject at point S by L, and a distance, from the optical axis of the lens 3b to a point on the image sensor 5b to which light from point S is incident, by X, the following equation is satisfied: EQU L=(B.times.F)/X
The distance X corresponds to a deviation of an image of the subject S focussed on the image sensor 5a from an image of the subject S focussed on the image sensor 5b. The subject distance L can be obtained by calculating the distance X as detected. A processing circuit 6 compares signals from the two image sensors 5a and 5b, while sequentially shifting one of the signals by one pixel at a time. When both the signals become the same within the range of a predetermined number of shifted pixels, the distance X can be obtained by multiplying a pixel pitch by the number of shifted pixels. The obtained distance X is correlated to the object distance L, so a lens drive circuit 7 is actuated to correspondingly drive a motor 8 and set a taking lens 9 at a lens position suitable for the distance X.
FIG. 8 shows a conventional integral range finding device as a unit. Lens sections 3a and 3b are integrally formed on a transparent acrylic resin lens plate 10 at positions spaced by a lens base length B. A lens holder or housing 11 has two tunnels of a quadrangle in section and is made of black colored plastics. On the front surface of the lens holder 11, the lens plate 10 is fixed with adhesive agent in the state of high positioning precision. On the back surface of the lens holder 11, an IC package 13 is fixed with adhesive agent. This IC package 13 has image sensors 5a and 5b covered with a cover glass 12. Reference pixels of the image sensors 5a and 5b are disposed spaced apart by a sensor base length C which is set equal to the lens base length B.
It is essential for the range finding device described above that the lens base length B representing the distance between the optical axes of the lens sections 3a and 3b be quite the same as the sensor base length C representing the distance between the reference pixels of the image sensors 5a and 5b. However, in an assembled state of the range finding device in a camera body, the lens plate 10, lens holder 11, and the silicon substrate forming the image sensors 5a and 5b expand or contract differently from each other at given temperature and humidity. Therefore, the spherical surfaces of the lens sections 3a and 3b may be deformed by a force caused by a difference of expansion or contraction between the lens plate 10 and lens holder 11. Furthermore, each element changes its mounting position as temperature or humidity changes, resulting in a relative change between the lens base length B and the sensor base length C. For example, since the thermal expansion coefficient of silicon substrate of the IC package 13 is small, the sensor base length C changes less with environmental conditions. However, resin is often used as the raw material of the lens holder 11 and lens plate 10, so that the lens base length B is likely to change greatly. A relative change between the lens base length B and sensor base length C is a main factor of distance measuring error, unable to maintain a reliable measurement precision.